Okay, I cannot answer this question based on experience, but I have the full metal POLIZIA one. I think the efficiency is the same, but polymer may be slightly better. Both of the guns should have similar, if not exactly the same, internals. Have you ever noticed that the barrel on all of KWA's pistols are polymer? That's because the friction is greatly reduced and is lighter to pull; thus making the gun more gas efficient. With this being said, maybe the polymer one is more gas efficient. On another note; KWA's polymer is not cheap at all. In other words, it reduces weight and is more comfortable. The MP9 and MP7 and other gas guns are polymer, the only one that would probably need a metal body is the Mac11, because it has more of an ABS type bodied feel (but that's a different story). Full metal is not always better. I have a polymer bodied M4, and when I put my attachments on, I can barely carry it (M203, Carry handle, carry handle rail, reflex sight). If it were full metal, I would not be able to hold it. Polymer on a pistol would still feel really firm and would not be as cold to the touch as metal. In my own opinion, I do prefer a full metal pistol though, not for quality, just because I like the way it feels. Both guns are pretty much the same, so it's up to you whether you want the polymer one or the metal one. I hope I answered all of your questions! (My team and I have the M93R, Mac11, Mac11, MP9, MP9R, Mark II, Mark III) Any questions, just ask and I'll get back to you ASAP~!